Magnetic recording media, such as audio tapes, video tapes, and computer discs and memory tapes, generally comprise a non-magnetic support having thereon a magnetic layer containing a ferromagnetic powder dispersed in a binder.
Magnetic recording media recently developed to meet the demand for high-density recording, such as Hi-8 video tapes, have achieved a high S/N ratio or noise reduction through manipulations such as size reduction of ferromagnetic powders, use of metal powders, increase in packing density, and smoothing of the surface of the magnetic layer.
Further, high definition VTR systems, such as "UNIHI", are required to be capable of rapid signal processing through increase in writing and reading speeds. For example, the head cylinder speed in a helical scanning system has reached 5400 RPM or higher. A magnetic recording medium which undergoes such high-speed sliding must have sufficient stability against damage from being run on a VTR. For this purpose, carbon black or other fillers having a Mohs' hardness of 8 or higher, called abrasives, have been used as disclosed in JP-A-59-193533, JP-A-59-186125, JP-A-59-191133, JP-A-59-189831, JP-A-59-63029, JP-A-63-168821, JP-A-63-168822, and JP-A-1-185821 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
It is desirable for achievement of high-density recording that the amount of fillers as well as that of binders be minimized so as to increase as much as possible the packing density of a ferromagnetic powder in the magnetic layer. The problem associated with the above-described recently developed magnetic recording media is that high packing is not achieved without a sacrifice in running stability, especially in resistance to damage.
On the other hand, while improvement in the smoothness of magnetic layers is needed to increase the output of magnetic recording media, a magnetic layer with a mirror surface tends to have poor running stability. That is, the coefficient of friction on running increases with an increase in surface smoothness, causing adhesion of the tape in the VTR. In this connection, the magnetic layer should contain fillers, especially carbon black in a large amount, to control smoothness. For example, JP-A-185821 suggests the use of coarse carbon black particles having an average diameter of at least 150 m.mu. to ensure running properties. However, use of such coarse particles unavoidably results in considerable reduction in packing and surface properties. While incorporation of coarse carbon black particles into a magnetic layer, even in a small proportion, is effective in maintaining running stability, there is a great spacing loss between the tape and the VTR head due to protrusion of the coarse carbon black particles in the magnetic layer. This leads to a noticeable decrease in output.
Further, coarse carbon black particles of 50 m.mu. or greater, added in an amount of from 3 to 20 parts by weight per 100 parts by weight of a ferromagnetic powder, are less effective in decreasing surface electrical resistance of the magnetic layer than the same amount of finer particles. Accordingly, the coarse carbon black particles should be used in a larger amount than fine particles. This means a decrease in packing density of the ferromagnetic powder, leading to a decrease in output of the magnetic recording medium. Where coarse carbon black is used in an amount of from 0.5 to 3 parts by weight per 100 parts by weight of a ferromagnetic powder only for the purpose of maintaining running stability, improvement in running stability, particularly a reduction in the coefficient of friction, will result. But protrusions of the coarse particles appearing on the surface of the magnetic layer make a spacing loss, causing output reduction or variation.
When fine carbon black particles having a particle size of from 5 to 35 m.mu. in a magnetic layer are used, the surface electrical resistance of the magnetic layer can be reduced more efficiently than when coarse carbon black particles are used. Thus, an addition amount of fine particles of from 0.1 to 5 parts by weight per 100 parts by weight of a ferromagnetic powder is sufficient. Within this addition range, the decrease in packing density is controllable. Addition amounts higher than 5 parts result in reduced packing density. However, even if the above fine carbon black particles are used in the above amount, it is still extremely difficult to maintain running stability. Specifically, the magnetic layer surface suffers from cracks due to an increase in frictional coefficient, causing an increase in dropout or a decrease in output due to clogging of the head.
JP-A-63-168821 describes oil (dibutyl phthalate, hereinafter "DBP") absorption of carbon black. According to the disclosure, carbon black particles having a DBP absorption exceeding 150 ml/100 g cause a reduction of S/N ratio while those having a DBP absorption of not more than 110 ml/100 g cause a reduction in output. That is, carbon black particles having too high or too low DBP absorption do not necessarily improve the electromagnetic characteristics of magnetic recording media.
Thus, even with the above-described techniques developed to date, it has been very difficult to satisfy both the requirements of high output and excellent running stability, because maintenance of running stability requires surface roughening of a magnetic layer, and efficient surface roughening inevitably requires use of coarse carbon black particles.